diff --git a/src/schema/validator.js b/src/schema/validator.js
index 73f53ee5..2ff56cab 100644
--- a/src/schema/validator.js
+++ b/src/schema/validator.js
@@ -69,21 +69,9 @@ export const validateAddon = (...args) => {
   return isValid;
 };
 
-const _validateLangPack = validator.compile({
-  ...schemaObject,
-  id: 'langpack-manifest',
-  $ref: '#/types/WebExtensionLangpackManifest',
-});
-
-export const validateLangPack = (...args) => {
-  const isValid = _validateLangPack(...args);
-  validateLangPack.errors = filterErrors(_validateLangPack.errors);
-  return isValid;
-};
-
 // Create a new schema object that merges theme.json and the regular
 // manifest.json schema.
-// Then modify the result of that to allow `additionalProperties = false`
+// Then modify the result of that to set `additionalProperties = false`
 // so that additional properties are not allowed for themes.
 // We have to use deepmerge here to make sure we can overwrite the nested
 // structure and can use object-destructuring at the root level
@@ -113,6 +101,34 @@ export const validateStaticTheme = (...args) => {
   return isValid;
 };
 
+// Like with static themes, we don't want additional properties in langpacks.
+// The only difference is, this time, there is no additional schema file, we
+// just need to reference WebExtensionLangpackManifest and merge it with the
+// object that has additionalProperties: false.
+const _validateLangPack = validator.compile({
+  ...merge(
+    schemaObject, {
+      types: {
+        WebExtensionLangpackManifest: {
+          $merge: {
+            with: {
+              additionalProperties: false,
+            },
+          },
+        },
+      },
+    }
+  ),
+  id: 'langpack-manifest',
+  $ref: '#/types/WebExtensionLangpackManifest',
+});
+
+export const validateLangPack = (...args) => {
+  const isValid = _validateLangPack(...args);
+  validateLangPack.errors = filterErrors(_validateLangPack.errors);
+  return isValid;
+};
+
 const _validateLocaleMessages = validator.compile({
   ...messagesSchemaObject,
   id: 'messages',
diff --git a/tests/unit/parsers/test.manifestjson.js b/tests/unit/parsers/test.manifestjson.js
index 3bc13220..ccd72345 100644
--- a/tests/unit/parsers/test.manifestjson.js
+++ b/tests/unit/parsers/test.manifestjson.js
@@ -1104,6 +1104,22 @@ describe('ManifestJSONParser', () => {
       );
       expect(manifestJSONParser.isValid).toEqual(false);
     });
+
+    it('throws warning on additional properties', () => {
+      const linter = new Linter({ _: ['bar'] });
+      const json = validLangpackManifestJSON({ content_scripts: ['foo.js'] });
+      const manifestJSONParser = new ManifestJSONParser(
+        json, linter.collector, {
+          io: { files: {} },
+        }
+      );
+      expect(manifestJSONParser.isValid).toEqual(false);
+      assertHasMatchingError(linter.collector.errors, {
+        code: messages.JSON_INVALID.code,
+        message: '"/content_scripts" is an invalid additional property',
+        description: 'Your JSON file could not be parsed.',
+      });
+    });
   });
 
   describe('static theme', () => {